Enhancing Detection ofPhytophthora cactorumin Naturally Infested Soil

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Abstract
Reliable methods were needed to detect Phytophtoora cactorum, one of the primary causal agents of Phytophthora crown rot of apple trees, in its natural soil environment. Apple or pear fruits, used in a bathing bioassay, were ineffective as detecting P. cactorum in naturally infested soil. Apple seedlings, cotyledons, and seedling leaf pieces were successful baits, bu cotyledons were the most sensitivet and efficient. Completely air drying soil subsamples and then remoistening them for several days before flooding and adding plant tissue baits (extended baiting procedure) greatly enhanced the detection of P. cartorum when compared to the standard, direct baiting procedure without prior manipulation of soil moisture. Bioassay incubation temperature, volume of water added to remoisten air-dried soil subsamples, and incubation period following remoistening all affected detection, but the photoperiod during incubation did not advantages of an extended baiting bioassay with apple cotyledons were: greater sensitivity than with pear or apple fruits or by direct baiting, readily available and inexpensive baits, formation of sporangia of P. cactorum directly on necrotic cotyledons, and lack of interference by contaminant Pythium species. Cotyledons also were colonized by zoospores of P. cambivora, P. citricola, and P. cryptogea, but not by those of P. megasprma, P. syringae, P. drechsleri, or an inidentified Phytophthora species. The extended bioassay procedure routinely has provide a relatively rapid and efficient means of detecting P. cactorum in a diversity of soils within and around New York apple orchards.